What is kilojoule?
A kilojoule is a unit of energy equal to 1,000 joules. It is commonly used in nutrition to express the energy content of food, as well as in thermodynamics and mechanical engineering.
Real-world uses
Kilojoules are the standard energy unit for food nutrition labelling in Australia, New Zealand, and parts of Europe (alongside kilocalories). A typical adult daily intake is about 8,700 kJ. They are also used in chemistry for reaction enthalpies and combustion energies.
History
The kilojoule became important in nutrition labelling as countries adopted SI-based energy reporting. Australia was among the first to mandate kJ on food labels in the 1970s. The EU requires both kJ and kcal on nutrition labels.
Common mistakes
Confusing kilojoules with kilocalories—1 kcal = 4.184 kJ. Food labels showing "2000 kJ" are about 478 kcal, not 2000 kcal. Also, some people incorrectly assume kJ and kW are the same thing; one is energy, the other is power.
What is electronvolt?
An electronvolt is a unit of energy equal to the work done on an electron by a potential difference of one volt, approximately 1.602 × 10⁻¹⁹ joules. It is used in atomic physics, particle physics, and quantum mechanics.
Real-world uses
The electronvolt is the standard energy unit in particle physics, atomic physics, and semiconductor science. X-ray photon energies are measured in keV; visible light photons in eV (about 1.8–3.1 eV); nuclear reactions in MeV; and particle accelerator energies in GeV or TeV.
History
The electronvolt came into use in the early 20th century with the development of quantum mechanics and experimental atomic physics. As physicists began measuring energies of electrons and photons, the eV provided a convenient unit far smaller than the joule but appropriate for subatomic-scale phenomena.
Common mistakes
Confusing the electronvolt with a unit of voltage—it is a unit of energy equal to the kinetic energy gained by one electron accelerated through a potential difference of one volt (1 eV = 1.602 × 10⁻¹⁹ J). Also mixing up the scale prefixes (keV, MeV, GeV, TeV) when discussing different physical phenomena.
When is this conversion used?
Nutritional information is labelled in kilocalories in some countries and kilojoules in others, making this conversion important for dietary planning and food science.
Worked examples
1 kilojoule = 6.241509e+21 electronvolt
1 electronvolt = 1.602177e-19 joule
How to convert kilojoule to electronvolt
To convert kilojoule to electronvolt, multiply the value by 6.241509e+21.
To convert electronvolt back to kilojoule, multiply by 1.602177e-22.
Measurement standards
The joule is the SI derived unit of energy, defined as one kilogram-metre-squared per second-squared (kg·m²/s²). The kilowatt-hour, equal to exactly 3.6 megajoules, is accepted for use with the SI by the BIPM for practical electricity metering.
Did you know?
A single bolt of lightning releases roughly 1 to 5 gigajoules of energy, yet only a small fraction reaches the ground as electrical current. Most of the energy dissipates as light, heat, and the thunder shockwave.
Quick reference: kilojoule to electronvolt
| kilojoule | electronvolt |
|---|---|
| 0.1 | 6.241509e+20 |
| 0.5 | 3.120755e+21 |
| 1 | 6.241509e+21 |
| 2 | 1.248302e+22 |
| 5 | 3.120755e+22 |
| 10 | 6.241509e+22 |
| 25 | 1.560377e+23 |
| 50 | 3.120755e+23 |
| 100 | 6.241509e+23 |
| 250 | 1.560377e+24 |
| 500 | 3.120755e+24 |
| 1,000 | 6.241509e+24 |
Common values
| kilojoule | electronvolt | |
|---|---|---|
| A food calorie (1 kcal) | 4.184 kilojoule | 2.611447e+22 electronvolt |
| A AA battery | 9.36 kilojoule | 5.842052e+22 electronvolt |
| Boiling a kettle | 420 kilojoule | 2.621434e+24 electronvolt |
| Daily human diet (2000 kcal) | 8,368 kilojoule | 5.222895e+25 electronvolt |
| 1 gallon of gasoline | 131,760 kilojoule | 8.223812e+26 electronvolt |
Available Energy units
More kilojoule conversions
- Convert kilojoule to joule
- Convert kilojoule to kilowatt hour
- Convert kilojoule to calorie (thermochemical)
- Convert kilojoule to kilocalorie (dietary)
- Convert kilojoule to watt hour
- Convert kilojoule to megajoule
- Convert kilojoule to electronvolt
- Convert kilojoule to British thermal unit
- Convert kilojoule to therm (US)
Assumption: calorie means thermochemical calorie, and therm means US therm.